1. Field of the Invention
An aspect of the invention relates to a method for high-resolution imaging of test objects by electromagnetic waves, in particular for inspecting individuals for suspicious items, in which the test object is illuminated with electromagnetic waves, and the scattered waves are received and analyzed using the synthetic aperture radar principle (SAR) to display an image of the test object.
2. Description of the Background Art
In order to inspect individuals or pieces of luggage (hereinafter referred to as test objects) for hidden dangerous items (weapons, explosives), imaging systems are known in which the test objects (individuals, pieces of luggage) are scanned with millimeter waves in order to detect suspicious items (U.S. Pat. No. 5,859,609). The advantage of these imaging systems is that nonmetallic items as well as metallic items can be easily detected due to their material-specific dielectric properties. Similarly, the use of radar-based millimeter wave imaging systems in the field of nondestructive testing of materials (NDT) has increased sharply.
To ensure reliable detection of dangerous items when used in the security field, or of defects in nondestructive testing of materials, the imaging systems require high spatial resolution. Furthermore, it is desirable, especially in applications in the field of security, for the system to be capable of scanning the largest possible area in a short period of time.
High resolution in one plane (lateral resolution) can be achieved with focusing elements, for example elliptical mirrors or dielectric lenses, which sharply focus the measurement signal on the surface of the test object. However, this type of focusing has inadequate depth of focus. High resolution is only provided in one plane, the focal plane. A further disadvantage is that when implementing fast scanning systems using large dielectric lenses or mirrors to concentrate the measurement signals, rapid motion of the large masses can be accomplished mechanically only with difficulty.
German patent application 10 2005 042 463, which is incorporated herein by reference, describes a method of the generic type in which a test object is successively illuminated along its circumference with millimeter waves, and the scattered millimeter waves are received and analyzed using the synthetic aperture radar principle (SAR) to display an image of the test object. A synthetic aperture is created by the means that the waves emitted by an antenna are first spatially concentrated, wherein the location of high concentration is manipulated such that it serves as a moving virtual antenna for SAR analysis.
In this imaging system using the SAR principle, it is known to use no other focusing components for beam forming beyond one antenna (monostatic) or multiple adjacent antennas (quasimonostatic), for example horn antennas, for transmitting and receiving radar signals reflected from the test object. The actual focusing here is performed after the fact by digital signal processing using the SAR principle. This type of data processing permits digital focusing of the raw data for any desired distance of the antenna from the test object. Additional focusing elements such as mirrors or lenses can be omitted, thus drastically reducing the mass of the millimeter wave sensors to be moved.
If a high lateral resolution in two dimensions (vertical and horizontal) is to be achieved with the SAR processing, then the phase center of each transmitting and/or receiving antenna must also be moved in two dimensions. This is achieved in a known manner by a linear motion in the horizontal and vertical direction. The motion of two axes (a raster scan, for example) would be too slow for a fast imaging system.